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Thread: Lowering electromagnetic noise for audio recording via microphone and cable

  1. #1
    Junior Member
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    Oct 2016
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    Lowering electromagnetic noise for audio recording via microphone and cable

    Hi Forum,

    What’s the best solution to keep, electromagnetic interferences as low as possible using Teensy/ Audio Adaptor Board and recording audio over a microphone via cable?

    Best solution of course without any cable:
    Soldering the Electret Condenser Microphone directly to the Audio Adaptor Board, as shown in the tutorials. https://www.pjrc.com/store/microphone.html
    Unfortunately regarding space concerns I need a wired microphone.
    https://www.dropbox.com/s/6brdutu55y...edMic.jpg?dl=0

    Test:
    I have connected the Electret Mic. to micin via a microphone-cable (1 wired cable with shield 1.5m length).

    Test without any electromagnetic noise source:
    Vpp max = 0.07V.
    https://www.dropbox.com/s/pewegxxygx...lence.jpg?dl=0

    Test with 100Watt halogen spot and dimmer near microphone, mic-cable and Teensy:
    Vpp max = 1.04
    https://www.dropbox.com/s/m5td1tmiuq...logen.jpg?dl=0

    Test with 100Watt halogen spot and a dimmer near microphone, mic-cable and Teensy holding the mic. in my hand:
    Vpp max = 2 (saturation)
    https://www.dropbox.com/s/iuamf674oj...logen.jpg?dl=0

    As you can see the electromagnetic influence during my tests was huge. The mic went even in saturation. The noise frequency was always 50Hz which is supply frequency in EU. I think the noise introduction was inductive.

    My questions:
    1.) Does the test reflect the reality or partly the reality, if I want that my project works also on stages, with huge halogen spots, instrument-amplifier and a lot of other stuff?

    If yes what can I do to improve the solution?

    1.) Convert the analog signal to digital signal via shortest connection and communicate digitally with Teensy.
    For example using ADC via I2C like: https://www.adafruit.com/product/1085
    But in this case I don’t know how to handle it with Teensy Audio System Design Tool https://www.pjrc.com/teensy/gui/inde...=AudioInputI2S ?
    Because I need to adapt the signal at code side… adding some filters, amps, frequency and amplitude detection etc..

    Or can you recommend me an ADC breakoutboard which I can easily implement with Audio System Design?

    2.) Using a XLR microphone and cable, but I don´t understand if hooking up to Teensy/ Audio Adapter Shield is possible. XLR has 3 connections. Teensy only 2.


    I`m very grateful, for any advise.

    Best regards

  2. #2
    It really depends on the microphone you are using, but most microphones have a pretty high output impedance which makes the line to the receiver a great antenna for noise.

    The best options are usually to add a transistor or opamp as close to the mic as possible to reduce output impedance and then put a grounded enclosure around that as a faraday cage.
    Have a look at the Alice Mic project which gives you a good old school example for a great microphone http://scotthelmke.com/alice-mic.html

    Another option is to use a mems microphone with a built in amp instead, a bit of a pain to solder, but works pretty well.

    Barring those, you will need to do a spectral analysis, but I would expect most of the noise to be at the 60hz point, so a band stop filter, either RLC in hardware, or FIR in software should help, problem is that male voice can be in that range, so not a great solution for audio recording. You would want to make it as narrow as possible if you go down that path to minimize distortion. If it's blue noise, you might be able to get away with a low pass filter, depending on the noise spectrum and your recording target.

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